The invention is related to a singlemode lightwaveguide-coupling element coupling two lightwaveguide sections together which are inclined at an angle towards each other and which have different widths. The different widths usually derive from a different radius of curvature of the lightwaveguides, e.g. a straight section and a bent section which are to be coupled. Another reason for different widths may be a different refractive index steps or contrast in the two lightwaveguides. When it is referred to lightwaveguides also the terms optical waveguides or photonic waveguides are used synonymously.
When designing an optical integrated circuit of any type, one is often confronted with the problem of linking two given waveguides in the optical circuitry by a waveguide, e.g. when designing a phased array grating. The paths followed by the waveguides are normally constructed or concatenated from a small family of curves, such as segments of lines, ellipses, circles or sinusoidal curves. For these path types the loss incurred via bending has been calculated in detail and some mask-generating software implements them.
Two types of loss occur when a straight waveguide section is coupled to a bent waveguide section with a constant radius of curvature. The so-called bending or radiation loss is present in the curved waveguide section and is a consequence of the departure from translational invariance in the guiding structure. The so-called transition loss occurs at the coupling position, hence when the path followed by the waveguide section has a discontinuity in its curvature. The bending radius of a straight section being infinite changes to the finite radius of the bent section. The magnitude of transition loss is related to the magnitude of the discontinuity.
Bending loss in a curved waveguide section is due to a radial outwards shift of the modal field profile. The may lead to higher energy dissipation in the cladding in comparison with a straight waveguide section and hence to an increased loss.
Transition loss can be seen as an offset and a shape deformation of the modal field profile which leads to the same effect as described for the bending loss.
U.S. Pat. No. 5,175,785 describes an optical waveguide with reduced bending loss, namely a waveguide which is in principle a multimode waveguide with means for attenuating modes higher than the primary mode, such that the waveguide acts as a virtual monomode waveguide.
In U.S. Pat. No. 5,278,931 it is proposed to use an inner core with a higher refractive index than the outer core as waveguide core for reducing bend loss in a singlemode optical waveguide fiber.
A publication dealing with tapered or curved coupling of waveguide sections with low loss is U.S. Pat. No. 4,787,689. There the refractive index profile is chosen variable in the vicinity of curves or tapers. Means for achieving this are prisms or lenses.
Two other strategies are known for minimization of bending and transition losses:
Widening the bent waveguide section is a way to decrease the bending loss in that it increases the confinement of the mode and thus reduces the leakage. The widening is however restricted in that it should not allow multimode propagation.
Inserting an offset in the curved waveguide section equal to the mode offset created by the bend, is supposed to ensure that the modal field profiles of the straight and the curved waveguide sections are better aligned and hence minimizes transition loss. Introduction of such offsets is discussed in the article xe2x80x9cBending Loss Reduction in Silica-Based Waveguides by Using Lateral Offsetsxe2x80x9d by Kito, Takato, Yasu and Kawachi in Journal of Lightwave Technology, Vol.13, No.4, April 1995, pp 555-562, and also in Patent DE3107112.
The optical arrangements shown in U.S. Pat. No. 5,243,672 have reduced loss because they comprise a combined widening and offset.
In U.S. Pat. No. 4,993,794 an integrated optic waveguide with a bend is disclosed. This waveguide is supposed to operate with multimode propagation. The waveguide is widened in the region of the bend such that the locus of maximum intensity in the waveguide oscillates from side to side so that the combined wavefront tilts to the left and then to the right. Smooth transitions are recommended to reduce losses.
The article xe2x80x9cA New General Approach to Optical Waveguide Path Designxe2x80x9d by F. Ladouceur and P. Labeye in Journal of Lightwave Technology, Vol.13, No.3, March 1995, pp 481-492, describes an adapted bending-loss reduction mechanism that relies on a continuous widening of the waveguide together with the reduction of transition loss through curvature adaptation. The motivation of this design lies in the fact that abrupt offset and widening create discontinuities and consequently losses. For transition loss minimization, a progressive bend is proposed which is combined with a widening for reducing bending loss.
It is an object of the invention to propose a singlemode lightwaveguide-coupling element for connecting two lightwaveguide sections which are inclined at an angle towards each other and which have a different width such that the losses are minimized. The width difference may be depending on a different bend radius and/or a different refractive index contrast.
An alternative design rule for creating this singlemode lightwaveguide-coupling element is disclosed.
With a singlemode lightwaveguide-coupling element according to claim 1, smaller bending radii can be achieved and lesser losses are introduced.
When the singlemode lightwaveguide-coupling element has a rectangular cross-section in the plane of curvature, the manufacturing process is easier and cheaper. Known processes, such as lithographic processes can be used for manufacturing.
When the singlemode lightwaveguide-coupling element has a non-constant bending curve in the plane of curvature and the tangents of adjacent waveguide sections are at least approximately identical, an exacter approach towards the curve of the dependence between waveguide width and bending radius can be achieved, such that the losses are further reduced.
The coupling losses are further reduced when the number of intermediate waveguide sections is chosen such that the approach xcex94xcex1i≈sinxcex94xcex1i is allowed, because thereby more points of the curve of the dependence between waveguide width and bending radius are used to determine the geometry of the singlemode lightwaveguide-coupling element which leads to an exacter guidance of the lightwaves.
The coupling losses are further reduced when the length (lv) is at least differentiable and decreasing dependent on the final width (Wvf) Then, the curve reflecting the dependence between waveguide width and bending radius is better approached.
The coupling losses are further reduced when the length (lv) is at least an approach to a logarithmical function of the final width (Wvf), since experimental data have proven that the logarithmic function represents a very good approach to the curve of the dependence between waveguide width and bending radius.
When the waveguide sections have a different refractive index step, the curve of the dependence between waveguide width and bending radius does not influence the bending radius but directly the length to be chosen according to claim 1. The bending radius can then be chosen as a mathematical help to determine the length of the intermediate waveguide sections.
Of course, also mixed forms of waveguides, incorporating bending as well as a variable refractive index step can be chosen. Then, the determination of the length dependent on the final waveguide width, according claim 1, leads to the desired result.
For connecting two singlemode waveguide sections which have different waveguide widths, the required widening of the waveguide decreases with increasing bending radius, for achieving a monoinodal and loss-minimized waveguide. The dependence between the radius r and the waveguide width w while is used while the abruptness of radius between the initial waveguide section and the final waveguide section is reduced. In the ideal case, the radius in the intermediate section changes exactly with the function of the dependence between the radius r and the waveguide width w.
When the waveguide width w is increased linearly with propagation distance, the relationship between the waveguide width w and the radius r of curvature leads to a logarithmic change of the bending radius r with the propagation distance in the bend. This fits to the assumption of exponential dependence between the waveguide width w and the bending radius r.
A singlemode lightwaveguide-coupling element is positioned between an initial waveguide section which there has a basic final width in a plane of curvature, and a final waveguide section which there has a basic initial width which is bigger than the basic final width by a width difference. The lightwave directions of both waveguide sections are inclined at a predetermined total angle towards each other.
Starting from the initial waveguide section, the lightwaveguide element comprises intermediate waveguide sections each of which at its end has a lightwave direction which is inclined towards the lightwave direction at its opposite end at an inclination angle, such that the sum of all inclination angles equals the predetermined total angle. Each of the intermediate waveguide sections has an initial width and a final width which is bigger than the initial width by a width difference. For each of the intermediate waveguide sections and for the final waveguide section, its initial width equals the final width of the directly preceding waveguide section. Each of the intermediate waveguide sections further has a length that is constant or steadily decreasing dependent on the final width.
By this geometrical rule, the coupling between the two waveguide sections is following the curve of the dependence between waveguide width and bending radius in that the radius difference or respectively the corresponding refractive index difference is bridged by intermediate waveguide sections with intermediate values of bending radius (or refractive index) while the length of these sections is determined by the dependence between waveguide width and bending radius. The more intermediate waveguide sections, the better the result.